<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    .a{
      width: 500px;
      height: 500px;
      background-color: yellow;
      display: flex;
      /*flex-direction: column;!* 控制子元素的横纵向排列*!*/
      /*flex-direction: column-reverse;*/
      /*flex-wrap: wrap;*/
      /*justify-content: space-around;*/
      /*flex-wrap: wrap;*/
      /*height: 200px;*/
      /*align-items: stretch;*/
      align-items: baseline;
    }
    .b{
      width: 100px;
      height: 100px;
      margin: 10px;
      background-color: #73AD21;
    }
  </style>
</head>
<body>
<div class="a">
  <div class="b">1</div>
  <div class="b">2</div>
  <div class="b">3</div>
<!--  <div class="b">5</div>-->
<!--  <div class="b">6</div>-->
<!--  <div class="b">7</div>-->
</div>
</body>
</html>
